Heavy-Duty Natural Gas Engines

Hannu Jääskeläinen

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• Introduction
• Spark Ignition Engines
• Direct Injection Compression Ignition Engines
• Dual Fuel Engines (Premixed Natural Gas, Diesel Pilot)

Introduction

Interest in natural gas for heavy-duty applications is driven by such factors as the potential for lower operating costs, compliance with tailpipe CO₂ emission requirements, and as a cost-effective solution to ultra-low NOx emissions.

Natural gas fuel costs are typically lower than for diesel fuel. If sufficient incentives are available to offset the higher capital cost of NG vehicles, or if the price differential between diesel and natural gas is sufficient for the purchasers of new vehicles to have a payback (usually in less than 2 years), natural gas vehicles become interesting for vehicle operators.

For manufacturers, the potential for lower tank-to-wheel CO₂ emissions is an important motivator to market natural gas engines, Figure 1 [4515]. The incentive is even more significant if the well-to-tank methane emissions (methane losses) is not a significant disincentive.

For achieving ultra-low NOx emissions such as California’s optional 0.02 g/bhp-hr, stoichiometric natural gas fueled SI engines have an advantage in that they can do so with relatively straightforward emission control system based on a three-way catalyst (TWC). The downside is that these engines have a relatively low brake thermal efficiency (BTE) of less than 39%. While diesel engines can achieve much better BTE, they require adding complexity to an already complex and expensive aftertreatment system through measures such as dual dosing urea SCR systems.

While natural gas engines are often claimed to have the potential for lower NOx and PM emissions, some have pointed out that several Euro VI natural gas heavy-duty trucks actually have higher NOx than their diesel counterparts [4521]. Natural gas vehicle proponents argue that this comparison should not be extended to tarnish all natural gas trucks [4522]. It should be noted that any difference in NOx and PM emissions between OEM natural gas and diesel vehicles is entirely attributable to differences in engine calibration and hardware choices rather than any inherent characteristics of the fuels. Vehicles using either fuel would still need to comply with applicable emission regulations. Manufacturers make engine hardware and calibration decisions based on numerous factors including available technologies, cost and expected sales volumes; the engines will still need to comply with applicable regulatory requirements. There is no incentive to over-comply for any regulatory requirements—especially if doing so would add cost and/or potentially reduce engine efficiency.

Spark Ignition Engines

Overview

Premixed charge spark ignition engines are very successful commercially due to their ability to achieve low emissions with relatively simple aftertreatment systems. Stoichiometric versions can achieve very low emissions of NOx while keeping methane emissions low with a TWC. Their relatively low thermal efficiency has been less of a concern in heavy-duty applications because the price of natural gas relative to diesel fuel has traditionally been low enough that significant fuel cost savings are still possible.

Lean burn SI engines, such as Doosan’s 11 L GL11K, have also been produced and typically use urea SCR aftertreatment to achieve low NOx emissions [4325]. However, control of methane emissions from lean burn engines is challenging and a viable commercial solution for Euro VI and EPA Phase 1 methane limits is not yet available. Thus, new lean burn SI natural gas engines are no longer commonly produced for the North American and European heavy-duty markets where relatively low limits on methane emissions exist.

Table 1 illustrates some details of two stoichiometric 12L natural gas engines and a 12 L diesel engine, all certified to meet 2017 EPA CO₂ limits [4467][4468][4469][4077][4177][3704]. The ISX12G natural gas engine meets 2010 NOx limits and generate CO₂ credits over both FTP and SET cycles. The ISX12N natural gas engine meets CARB’s optional 0.02 g/bhp-hr limit and also generate CO₂ credits over both FTP and SET cycles. The X12 diesel engine meets 2010 NOx limits and generate CO₂ credits over the FTP cycle but requires credits for the SET cycle. One obvious observation is that natural gas offers the potential for significantly lower NOx and CO₂ with lower technical complexity compared to the diesel engine. However, challenges remain including a lower efficiency, higher methane emissions and lower power/torque density.

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